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Materials 2013 , 6 , 5171-5198; doi:10.3390/ma6115171

materials ISSN 1996-1944

www.mdpi.com/journal/materials Review

Recent Development of Flax Fibres and Their ReinforcedComposites Based on Different Polymeric Matrices

Jinchun Zhu 1,*, Huijun Zhu 2, James Njuguna 3 and Hrushikesh Abhyankar 1,*

1 Centre of Automotive Technology, Cranfield University, Cranfield, MK43 0AL, UK;2 Cranfiled Health, Cranfield University, Cranfield, MK43 0AL, UK; E-Mail: h.zhu@cranfield.ac.uk3 Institute for Innovation, Design and Sustainability, Robert Gordon University, Aberdeen

AB10 7GJ, UK; E-Mail: j.njuguna@rgu.ac.uk

* Author to whom correspondence should be addressed: E-Mails: j.zhu@cranfield.ac.uk (J.Z.);h.a.abhyankar@cranfield.ac.uk (H.A.); Tel.: +44-123-475-0111 (H.A.).

Received: 2 September 2013; in revised form: 26 September 2013 / Accepted: 14 October 2013 / Published: 12 November 2013

Abstract: This work describes flax fibre reinforced polymeric composites with recentdevelopments. The properties of flax fibres, as well as advanced fibre treatments such asmercerization, silane treatment, acylation, peroxide treatment and coatings for theenhancement of flax/matrix incompatibility are presented. The characteristic properties andcharacterizations of flax composites on various polymers including polypropylene (PP) and polylactic acid, epoxy, bio-epoxy and bio-phenolic resin are discussed. A brief overview isalso given on the recent nanotechnology applied in flax composites.

Keywords: flax composites; mechanical properties; modifications

1. Introduction

As a result of the growing environmental awareness (e.g., increased pollution, increasing demandfor biodegradable materials, material need for CO2 neutrality and low greenhouse gas emissions, newenvironmental laws and regulations), manufacturers and scientists are keen to study novelenvironmental friendly materials. Over the last decade intensive research and development has been

carried out in order to develop powerful composites using natural fibres, offering good bio-degradabilityand sustainability. A biodegradable material will slowly undergo biodegradation by surroundingmicroorganisms, bacteria, and exposure to the elements and hence could provide solutions to end-of-life

OPEN ACCESS

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issues after service life [1]. Nowadays, the fibres resulting from wood, animals, leaves, grasses andother natural sources are commonly used as reinforcement in composites used for various applications,like automotive (interior and exterior), building, ship, packaging etc. , due to their unusual propertiescompared to other synthetic fibres. Advances in manufacturing techniques in natural fibre-reinforcedcomposites have allowed the car industry to utilise these composites in interior trimmings.

Besides the environmental benefits, compared to glass fibre composites, the natural fibre reinforcedcomposites with the equivalent performances have higher fibre content, resulting in less pollution fromsynthetic polymer matrix, and much lighter weight, reducing the amount of driving fuel in automotiveapplications. Table 1 summarises the general reasons for using natural fibres to reinforce polymers. Yuand his co-workers [2], Saheb and Jog [3], Avella [4], and Holbery and Houston [5] have reviewednatural fibre composites with their classifications, properties and potential applications. The naturalfibres are divided into the groups of animal (wool and silk), mineral (asbestos) and plant/vegetable(bast, leaf, seed, wood, and grasses) as shown in Figure 1. The development of natural fibrecomposites is limited due to several issues: (1) the thermal degradation of natural fibres could decreasethe mechanical properties (toughness and bending strength), result in poor organoleptic properties(odour and colour) and possible production of volatiles at processing time over 200 C; (2) the highmoisture content of natural fibres, especially cellulosic fibres, could lead to poor dimensional stabilityand process-ability, and porous issues; (3) the composites exposed outdoors may bio-degrade byultraviolet light; (4) the dispersion of natural fibres is affected by the strong inter-fibre bonding; and(5) the incompatibility between hydrophobic polymer matrix and hydrophilic natural fibres [3].

Table 1. Comparison between natural fibres and synthetic fibre [6,7].

Fibre Advantages Disadvantages

Natural fibreBiodegradable Inhomogeneous quality

Low density/price Dimensional instability

Synthetic fibreMoisture resistance Difficult in recycle

Good mechanical properties Relative high price

Among the abundance of natural fibres, bast fibres (flax, hemp, jute etc. ) are commonly used incomposite preparation. Summerscaleset al . [8], Anandjiwala and Blouw [9], and Cao et al. [10] havereviewed the research and development of bast fibres, derived from the outer cell layers of the plantstems.Bast fibre stems have a high Youngs modulus up to 140 GPa, comparable to aramid fibres. Thetypical mechanical properties of the important bast fibres are shown in Table 2. The mechanical meritstogether with their biodegradability make bast fibres to be increasingly considered as reinforcementsfor composites in the sustainable future. Due to the environmental benefits and attractive performanceof flax fibres from natural sources, the use of flax fibres to reinforce polymeric matrices has beensignificantly developed for various applications (vehicle, transport, construction etc. ) in recent years.

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2. Flax Fibres as Reinforcement

2.1. Origin of Flax Fibres

Flax fibres come from the flax plant, one species of Linum usitatissimum is bred, and is widelycultivated in West Europe where the daily temperature is generally below 30 C [17]. The flax planthas a life cycle of 90 125 days including vegetative, flowering and maturation periods [17]. Thediameter of the flax stem is in the range of 1 2 mm, with a height of about 80 cm. It can be seen fromFigure 2 that there are three layers bark, bundle and xylem in the flax stem. The outer layer of barkfunctions as a protective cover from external attacks except for the penetration of water and othernutrients [17].

Figure 2. Composition and cross section of flax stems. Adapted with permission from [17].

Copyright 2013 by Elsevier.

Figure 2 represents the structure and composition of the origin flax stems. During the fibre processing, the bark, however, together with xylem is eliminated to leave fibre bundles consisting ofelementary fibres. Technical fibres are extracted by partially separating the fibre bundles in the flax plant and can be as long as the stem length (approximately 1 m). Unlike the technical fibres, the lengthof elementary fibres varies between 2 cm and 5 cm, and the diameter is about 19 25 m. The polyhedron shapes (five to seven sides) help pack the elementary fibres together [18]. The elementaryfibres have primary and secondary cell walls, both of which are cellulose material. Cellulose fibrils(diameter between 0.1 m and 0.3 m) are embedded in concentric lamella composed of about 2% pectins and 15% hemicellulose which contribute to the thermal degradation and water uptake of thefibres [19]. They can be highly oriented with the fibre axis and thus crystallised in the cell walls to provide high tensile strength [18].

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2.2. Properties of Flax Fibres

2.2.1. Tensile Properties

It has been reported that there is no large scale plastic deformation of flax fibres in stress-strain behavior as the amorphous regions between fibril are oriented [16]. Baley et al. [20] found alongitudinal Youngs modulus of 59 GPa and transverse modulus of approximately 8 GPa. Theclamping length of fibres, however, has a great effect on the tensile strength. Bos [16] found that thetensile strength of the technical fibres has a plateau value of 500 MPa, which increases significantly below the clamping length of 25 cm. Two main reasons are mentioned in his report: (1) less criticalflaws; (2) changes in failure mechanism. Flaws, such as kink bands resulting from the isolation process etc. , are reduced by decreasing fibre length to increase the fibre strength. On the other hand, for a largeclamping length, failure takes place through the weak interphase, while the cracks can only propagate

through cell walls at a clamping length below the elementary fibre length lying between 20 mm and50 mm as seen in Figure 3. The mean value of the tensile strength of technical fibres is only 57% ofthe elementary fibre strength, 1522 440 MPa, due to the bulk effect.

Figure 3. Schematic representation of failure mechanism of flax fibres. At 25 mmclamping length, elementary fibres slip over each other. At 3 mm, cracks run through thecross section of the elementary fibres. Adapted with permission from [16]. Copyright 2013 by Eindhoven University.

In some areas, flax fibres are competitive to glass fibres, and hence are reasonably acceptable asreplacement. Except for the specific properties of flax fibres as shown in Table 3, three other reasonshave been stated to make the application of flax fibre more attractive: (1) cheaper than glass fibres;(2) less toxic; (3) high strength to weight ratio. Normally, the flax fibres have a relatively low pricecompared to glass fibres. In addition, glass fibres are suspected of causing lung cancer, but there is nosuch problem for natural fibres [16]. The thermal recycling of the flax fibres (burning of flax fibreswith few slags left) hasa great advantage over glass fibres.

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Table 3. Tensile properties of glass and flax fibres [16].

Property E-glass Flax fibresDiameter (m) 8 14 10 80Density (g/cm3) 2.56 1.4

E-modulus (GPa) 76 50 70Tensile strength (GPa) 1.4 2.5 0.5 1.5

Specific E-modulus (Gpa/gcm3) 30 36 50Specific tensile strength (GPa/gcm3) 0.5 1 0.4 1.1

2.2.2. Compressive Properties

Similar to glass fibres, the compressive strength of flax fibres could be measured by the elastic looptest (Figure 4). Flax fibres usually fail in the top of the loop because of the highly oriented structure.

The compressive strength, c, is calculated from [16]:

(1)

where the E c is the elastic modulus for both tensile and compressive;d the fibre diameter;C c the pointof failure. Fibre samples were tested by Bos [16] and a compressive strength range of 830 1570 MPawas obtained.

Figure 4. Loop test for compression. The ratio c/a changes at failure point.

2.2.3. Physical PropertiesThe degradation of flax fibres is a crucial aspect in the development of natural fibre composites and

thus has a bearing on the curing temperature in the case of thermosets and extrusion temperature inthermoplastic composites. Cellulose is the main component of natural fibres, and thus controls themajor degradation behavior of flax fibres. The degradation routes for cellulose upon heating arediscussed in the literature [21]. The glycosyl units are decomposed at low temperature, followed bytheir depolymerisation at high temperatures. Then the formed substances like levoglucosan decomposeinto gas at higher temperatures.

The effect of thermal degradation on mechanical properties of flax fibres was investigated byGassan and Bledzki [22]. They placed the untreated flax fibres in a laboratory oven between170 210 C for a maximum of 120 min. Then the tenacity of flax fibres was measured by a tensile test.

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The tenacity decreased slightly below 170 C, whereas there was a rapid decrease of tenacity and polymerisation degree at temperatures above 170 C. The behavior strongly depends on the exposuretime and temperature. Additionally, a slight increase of crystallinity was observed after heat treatment,as a result of chain scissions.

Stamboulis, Baillie and Peijs [23] demonstrated that moisture absorption varied between Duralinflax fibres and Green flax fibres. Duralin fibres absorbed less water and retained a smoother fibresurface after fibre separation than Green flax fibres. The tensile strength of Duralin flax fibresincreased to a maximum tensile strength at a humidity of 66% and decreased afterwards. The reason isthat water plasticised the fibres at low absorbed water content but this effect was less important at largemoisture content.

2.3. Development of Surface Treatment on Flax Fibres

To improve the adhesion between hydrophilic flax and hydrophobic polymer matrix, many studieson using chemical or physical treatments have been published [24 27]. Some typical treatment routesfrom published articles are collected in Table 4. Common treatments include mercerization, dewaxing,silane treatment, acrylation, peroxide treatment, coatings, and impregnation with a dilute epoxy [28 30].

Table 4. Different treatments of flax-reinforced composites.

Fibre/matrixChemicaltreatment

Conditions Effect on properties Reference

flax/PP esterification 10 wt % MA, 25h, 50 C highest flexural and tensile

strength[26]

flax/phenolic esterification 25 wt % MMA, 30min,210 W more moisture retardant [27]

flax/epoxy alkali treatment 5 wt % NaOH, 30 min tensile strength 21.9%;flexural strength 16.1% [31]

flax/epoxy alkali treatment 4 wt % NaOH, 45 s transvers strength,30% increment [32]

flax/polyester silane treatment 0.05 wt %, 24 h hydric fibre/matrix interface [33]flax/pp esterification MA-PP coupling agent interphase compatibility [34]

*MMA methylmethacrylate; MA maleic-anhydride.

2.3.1. Silane Treatment

Coupling agents usually improve the degree of crosslinking in the interface region and offer a perfect bonding. Among the various coupling agents, silane coupling agents were found to be effectivein modifying the natural fibre-matrix interface. Proper treatment of fibres with silane can increase theinterfacial adhesion to the target polymer matrices and improve the mechanical performances of theresulting fibre/polymer composites. Silane is hydrolyzed forming reactive silanols and is then adsorbedand condensed on the fibre surface (sol-gel process). The hydrogen bonds formed between theadsorbed silanols and hydroxyl groups of natural fibres may be further converted into covalent bonds by heating the treated fibres at a high temperature (see Figure 5).

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Figure 5. Grafting of silanols on flax fibre surface (redrawn from Singhaet al . [35]).

Xie and his co-workers [13] reviewed silane coupling agent modification to natural fibrecomposites and found out improvements in strength, moisture absorption and fungal resistance for UPand epoxy composites. The suitable silane modification for fibres in epoxy composites is aminopropyltriethoxy siloxane (APS) and for methacryloxypropyl trimethoxysilane (MPS). APS solution (3%)combined with alkali treatment was found to provide better moisture resistance [35].

2.3.2. Acetylation

Acetylation is a well-known esterification method originally applied to wood cellulose to stabilizethe cell walls against moisture, improving dimensional stability and environmental degradation. Inlignocellulosic material the acetic anhydride reacts with more reactive hydroxyl groups (OH) in ligninand hemicellulose (amorphous material), whereas the hydroxyl groups of cellulose (crystallinematerial) prevent the diffusion of the reagent and result in a low extent of reaction [36]. Tensile andflexural strengths of flax/PP composites were found to increase with increasing degree of acetylationup to 18% [37].

2.3.3. Anhydride Treatment

Anhydride treatment is usually carried out by utilizing maleic anhydride or maleated polypropylene (PP) or polyethylene (PE) in a toluene or xylene solution, where the fibres are

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immersed for impregnation and reaction with the hydroxyl groups on the fibre surface. Hugues,Carpenter and Hill [24] reported that methacrylic and propionic anhydride modification showed amarked effect on yield properties (yield point, on set of yield etc. ) of flax composites. There are twomodification mechanisms used: (a) reactive vinylic group introduced by methacrylic anhydride (MA);and (b) hydrocarbon coating on the surface to improve the hydrophobicity by propionic anhydrides(PA) (see in Scheme 1). The trend of bond strength was observed as unmodified < PA < MA, and wasrelated to the work of fracture trend: MA < PA < unmodified due to the fact that the debonding behavior was inhibited by good bonding strength. The treatment of natural fibres with MAPPcopolymer decreased the polar component of the surface energy to a similar value of PP and henceimproved the wettability [26].

Scheme 1. Schematic presentation of the reaction between OH groups of flax fibres and(a) methacrylic; (b ) propionic anhydrides.

2.3.4. Mercerization/Alkali Treatment

Alkali treatment of natural fibres, also called mercerization, is the common method to producehigh-quality fibres. Mercerization has an effect on the chemical composition of the flax fibres, degreeof polymerization and molecular orientation of the cellulose crystallites due to cementing substanceslike lignin and hemicellulose which are removed during the mercerization process [34]. Alkali

treatment also converted the crystalline form of cellulose I into cellulose II [32]. The extent of thistransformation could be to some extent reflected by the intensity ratio of the stretching modes ofsymmetric (C O C) and asymmetric (C O C) through FT Raman spectroscopy [25]. Mechanical properties of polystyrene composites reinforced with chemically treated flax fibre were investigatedand it was observed that mercerization of flax fibres improved the mechanical properties of polystyrene composites [38].

The changes in surface morphology and chemical compositions, along with mercerization greatlyinfluence the thermal degradation of flax fibres and hence the processing temperatures. Bledzki et al. [39]reported that the degradation temperature of flax fibres increased from 319 C to 360 C afteracetylation (34% acetylation). It was also stated that the thermal stability of flax fibres increased afteralkali treatment due to the composition change on the removal of lignin and hemicellulose [40].

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2.3.5. Enzymatic Treatments

Enzymes are an increasingly interesting option as such or when combined with chemical andmechanical methods for modification and processing of biomaterials. This is due to the fact that

enzymes are highly specific and efficient catalysts and they work in mild, energy-saving conditions.Oxidative enzymes, such as laccases or peroxidases, can be used to activate and further functionaliselignocellulosics [41]. The primary reaction of laccase is the oxidation of phenolic hydroxyls to phenoxy radicals in the presence of oxygen. Laccases can thus be used to activate lignin, lignans, anddifferent types of lipophilic extractives present in the complex lignocellulosic materials [42]. It wasfound that the lignin content of single cellulose fibres decreased from 35% to 24% withlaccase treatment.

Research activities in the use of laccase in various application areas have recently been reviewed byKudanga et al. [43]. The laccase-catalysed modification can be used to tailor the properties of variouslignocellulosic materials, including flax fibre materials based on the application needs [44]. Natural phenols, such as syringaldehyde, acetosyringone and p-coumaric acid, in combination with laccasetreatment, were recently developed to graft on the flax fibres and offered antimicrobial properties [45,46]. Lauryl gallate (LG), a hydrophobic compound with the strongest internal sizingeffect, was grafted onto cellulosic fibres, and the results showed a significant reduction in water penetration [47,48].

2.3.6. Other Treatments

There are still a number of available pre-treatments, such as benzoylation [49], etherification [50],isocyanate treatment [51], peroxide treatment [38], sodium chlorite [52] and stearic acidtreatment [53]. In benzoylation treatment, benzoyl chloride is most often used in fibre pretreatment.The inclusion of a benzoyl group in the fibre is responsible for the decreased hydrophilic nature of thetreated fibre, decreasing its water absorption, but also increasing its strength properties. For themodification of cellulosic fibres by etherification sodium hydroxide plays an important role in forminga charged intermediate species with the fibre, which allows faster nucleophilic addition of epoxides,alkyl halides, benzyl chloride, acrylonitrile, and formaldehyde. The isocyanate group can react with thehydroxyl groups on the fibre surface forming covalent bonds, thus improving the interface adhesion.Organic peroxides tend to decompose easily to free radicals (RO.), which further react with thehydrogen group of the matrix and cellulose fibres. Sodium chlorite (NaClO2) is used usually in bleaching fibres; however, it could delignify lignocellulosics. Fibre treatment of stearic acid(CH3(CH2)16COOH) in ethyl alcohol solution was reported in that this treatment removednon-crystalline constituents of the fibres, thus altering the fibre surface topography. It could beinteresting to add a conclusion: these treatments are not very eco-friendly and the better way to usenatural fibres is to use them as received without chemical treatments.

3. Properties and Characterization of Flax Polymeric Composites

Like common polymer composites, either thermoplastics or thermosets could be used with flaxfibres and offer various mechanical properties. From the matrix point of view, thermoplastic matrices

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like PP and PE are ductile, easy to process and simple to recycle. On the other hand, processingtemperature and time must be properly controlled so as to decrease the viscosity for suitable wettingand not degrade the fibres. The final cross-linked thermoset structure of low viscosity of the monomershighly increased the mechanical and thermal properties. However processing is more demanding andrecycling is restricted. Concerning the increasing needs of recyclable materials, bio-degradable polymeric matrices (polylactic acid (PLA), soy protein epoxy and tannin phenolic resin) have attractedmore and more attention from academics and industrialists. Table 5 shows the collected mechanical properties of flax-reinforced composites based on different matrices. Except for mechanical performance, the physical information of flax fibres and associated composites, could be obtainedthrough various approaches, such as DSC (differential scanning calorimetry) [54], DMA (dynamicmechanical analysis), TGA (thermogravimetric analysis) [54], X-ray diffraction [34], SEM (scanningelectron microscopy) [54,55] and FTIR (Fourier transform infrared spectroscopy) [53,56].

3.1. Thermoplastic Polypropylene Based Flax Composites

Many studies [37,57 60] have concentrated on flax/thermoplastic composites and have providedvaluable information. From the investigations of Van de Velde and Kiekens [37], PP is the mostsuitable thermoplastic matrix for flax-reinforced composites due to its various advantages, such as lowdensity, low thermal expansion, good resistance to water and recyclability. The adhesion betweenhydrophilic flax fibres and hydrophobic PP is the problem of most concern, modification technologiestherefore have been applied to improve it. Boiling of flax and use of chemicals (e.g., maletic acid) was proven to be good for adhesion modification in order to increase mechanical properties [61 63].Garkhail, Heijenrath and Peijs [64] prepared flax/maletic-anhydride grafted PP (MA-PP) composites by two production methods, called film-stacking and paper-making process. The effect of fibre lengthon composite stiffness and tensile strength showed very little agreement with the model predictionsshared with other scientists [65,66]. The critical length was thought to be reduced by the addition ofMA-PP, compared to PP/flax. Fibre volume however had a significant effect on the final mechanical properties of composites. The influence of physical structure of flax fibres on mechanical propertieswas investigated by Van Den Oever, Bos and Van Kemenade [67]. 40 vol % hackled and 40 vol %scotched fibre/PP were used to compare with the theoretical predictions. The results indicated thatcombing the flax fibres (hackled fibres) removed some weak lateral bonds with detriment to tensileand flexure strength. The compressive behavior of composites is also related to the presence of kink bands which can be removed by combing [68].

The hydrophilic character of bio-composites cannot be neglected, resulting from which waterabsorption/ageing plays an important role in degradation and decrease of mechanical properties.Recent studies [23,69 71] have reported that the effect of water uptake in bio-composites limits theiroutdoor applications. In general, there are three ways to understand the termwater absorption:(1) water diffuses directly into the matrix; (2) through interphase matrix/reinforcements; and (3) byimperfections, like pores and cracks. A study of moisture absorption and environmental durability of

flax (Green and Duralin)/PP composites was conducted by Stamboulis and his co-workers [69]. Atroom temperature, composites absorbed water towards an equilibrium and then becametime-independent. The plotted curve of moisture content versus root time (Figure 6) could be explained

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by Ficks law. Green flax/PP composites were clearly more sensitive to water than Durbin flax ones, meanwhile the addition of MA (maleic-anhydride)-PP lowered the initial water uptake rate with littleeffect on the maximum moisture content. In terms of mechanical properties, the moisture contentaffected stiffness of flax/PP composites more than tensile strength (Figure 7). The stiffness increasedsomehow at low moisture content due to the filled interfacial gap by swelling flax fibres, while itdecreased significantly at 7% moisture content. It is more likely because of fibre degradation caused by fungus development, not a micro cracking mechanism in a brittle matrix.

Figure 6. Moisture content of flax/PP composites as a function of time. Adapted with permission from [69]. Copyright 2013 by Springer.

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Table 5. Testing data of mechanical properties of flax reinforced composites.

Fibre/matrix Processing methodTensile strength

(MPa)Tensile modulus

(GPa)Flexural strength (MPa) Impact strength (kJ/m 2) Reference

flax/bio-thermoset(MSO)compression

moulding 50 120 6 15 180 (max) [72]

flax/bio-thermoset(MMSO)

compressionmoulding

50 120 7 15 201 (max) [72]

Arctic Flax/Epoxy(50:50)

resin transfermoulding 280 40 [73]

plain woven flax/epoxy hand lay-up 78 100 (MPa/g cm3) 17 35 (kJ/m2/g cm3) [74]

plain-woven flax/thermosetcompression

moulding280 32 250 15 (Charpy) [75]

flax yarn/SPC resins pultrusion 298 4.3 117 [19]flax/Lactic acidresins(70:30)

compressionmoulding

62 9 96 [76]

flax/PLA injection moulding 40 55 3 6 9 11 (Charpy) [77]

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Figure 7. Moisture content versus mechanical properties: (a) stiffness; (b) strengthAdapted with permission from [69]. Copyright 2013 by Springer.

(a)

(b)

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3.2. Thermoset-Epoxy Resin Based Flax Composites

The mechanical performances of epoxy based flax composites have been widelyinvestigated [73,78 80]. Hughes et al. [24] produced unidirectional flax/epoxy composites and

directed their investigation towards tensile deformation behavior. The work revealed the non-linearstress-strain relationship under tensile loading with respect to the flax/matrix adhesion and the presence of kink bands. Muralidar [81,82] studied epoxy composite reinforced by flax in the form ofhybrid performance (plain weave fabric and rib knitted structure) through a lay-up method. He pointedout that the compressive properties were mainly contributed by the matrix whereas the tensile properties of woven composites were highly influenced by the flax volume fraction in thetension direction. The effect of woven flax fabric on fracture toughness of flax/epoxy composites wasreported by Liu and Hughes [83]. The well-packed fibres in the textile lead to a high fibre volumefraction and hence the improvement of fracture toughness up to 9 MPam1/2 compared to pure resin(about 1.8 MPam1/2). Additionally, the results strongly depended on the testing directions (weft andwarp) with different fibre densities. Oksman [73] found that Arctic Flax /epoxy composites hadoutstanding mechanical properties (e.g., maximum tensile strength of 280 MPa) and presented a betterspecific modulus of 29 GPa/gcm3 than that of glass/epoxy composites. Liang and his co-workers [78]compared fatigue behaviours between glass fibre/epoxy and flax fibre/epoxy composites. Theyreported that flax composites had the advantages of a relatively stable modulus under cycling load overglass fibre-reinforced composites.

Like all the natural fibre composites, environmental durability plays a very important role inflax/epoxy composites. Assarar and his co-workers [70] reported the influence of water on the ageing properties of epoxy composites with 11 flax unidirectional plies. The failure stress decreased around13% after the first 1-day water immersion. Then the reduction of failure stress was only an extra 2% between 1 day and 20 days of immersion. From Figure 8, a 30% decrease of normalisedYoungsmodulus of flax/epoxy in the first 10 days showed a much worse result from water ageing than glassfibre composites. Hewman [84] tested the water damage of plain-weave flax/epoxy composites (eight plies) through the wet-dry cycle procedure. The tensile strength after the first wet-dry cycle was foundto be 89% of the unconditioned value, and dramatically dropped to 16% after the fourth cycle.Penetrated water expanded the flax fibres and resulted in matrix/technical fibre gaps (SEM images in

Figure 9) after drying due to the different shrinkage degrees of fibres and matrix. The increasednumber of wet-dry cycles enlarged these gaps (auto-accelerative process) and consequently weakenedthe load transfer.

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Figure 8. NormalisedYoungs modulus (modulus/modulus at time = 0) versus normalisedimmersion time (test time/saturation time). Adapted with permission from [70]. Copyright2013 by Elsevier.

Figure 9. Scanning electron microscopy (SEM) images of flax/epoxy composites:(a) origin specimen; and (b ) after eight wet-dry cycles. Adapted with permission from [84].Copyright 2013 by Elsevier.

3.3. Bio-Degradable Poly(Lactic Acid) (PLA) Based Flax Composites

The natural fibre (e.g., hemp, flax, jute and sisal) reinforced composites to replace glass-reinforced

composites have been well-established for several years because of increasing environmentalawareness. Nevertheless, due to the petroleum-based polymer matrices (e.g., PP and epoxy), the

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composites are not fully degradable, leading to difficulty in the recycling process. Hence, a newgeneration of fully bio-materials needs to be developed.

To improve the sustainability and eco-efficiency, bio-degradable poly(lactic acid) has beenincreasingly used together with flax fibres for fully bio-composites [4,85 87]. PLA can be derivedfrom corn starch, sugarcane etc. , while it is also able to be synthesized, especially for industrial production. Akesson et al. [76] produced and studied fully biodegradable composites from PLA andflax fibres. The mechanical properties of final composites were increased by varying the fibre contentfrom 40 wt % to 70 wt %, but drastically reduced with a fibre ratio of 75 wt %. The storage modulus ofPLA composites with 70 wt % flax fibres was 9.32 GPa and 3.29 GPa at 20 C and 140 C,respectively. The humidity ageing tests showed that the tensile modulus was reduced from 9 GPa to2.5 GPa after 1000 h exposure time at 95% humidity and 38 C, meanwhile the tensile strength wasreduced by about 70%. It was demonstrated by Bax and Mssig [77] that the flax/ PLA composites hada higher Youngs modulus of 6.31 GPa than Cordenka/PLA composites. The impact strength increasedto 11 kJ/m2 with a fibre mass fraction of 30 wt %. Oksman et al. [87] manufactured flax (long heckledfibres)/PLA composites by compression moulding and then compared them to the commercial flax/PPcomposites used for automotive panels. The composites with 40% flax content showed good fibredispersion and over 50% higher tensile modulus up to 7.3 GPa than flax/PP composites (as seen inFigure 10). The DMA results indicated that the reinforcement of flax fibres increasedT g from 50 Cfor pure PLA to 60 C for the composite, and presented a cold crystallization at 80 C. For theadhesion bonding measurement of flax/PLA composites, Le Duigou and his co-workers [88] designeda microbond test to estimate the interfacial shear strength. With decreasing thermal treatment rate, the

shear strength increased from 33 MPa for 93 C/min to 38 MPa for 1.5 C/min, as a result of thethermal residual stress from the crystallization.

Figure 10. Tensile properties of flax/PLA compared to flax/PP composites. Adapted with permission from [87]. Copyright 2013 by Elsevier.

3.4. Bio-Epoxy Resin Based Flax Composites

Vegetable oil-derived renewable epoxy resin is a potential substitute of petroleum-based resin forflax composites to be used in automotive and construction applications. Bio-epoxy resins have beendeveloped with flax fibre reinforcement [72,75,89 91]. Flax yarn and flax woven fabric reinforced soy

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protein concentrated resins (SPC) were prepared by Huang and Netravali [19] for the comparison oftensile and flexural properties. Two layers of unidirectional flax yarns were impregnated with resinsolution by winding fabrication, and then air cured at 35 C for 24 hours, followed by a 25 minute hot pressing at 120 C of 8 MPa. Flax fabric composites were made of four resin-coated flax fabrics curedat the same hot pressing condition. Flax yarn composites showed the highest tensile strength of298 MPa and flexural strength of 117 MPa, while for flax fabric composites, failure stresses of 62 MPaand 83 MPa were observed in the warp and weft direction, respectively. The reinforcement of both flaxyarns and flax fabrics results in high failure strain and high toughness.

Adekunle et al. [72] prepared bio-epoxy composites reinforced by hybrid non-woven and wovenflax fabrics. MMSO (methyacrylic anhydride modified soybean oil) concentrate bio-epoxy resins andMSO (methacrylated soybean oil) resin were applied. The composite laminates consisted of threenon-woven flax mats sandwiched between four woven fabrics (0, 45, 90 orientation) with differentstacking sequences. The fibre configuration of [0

4/N

3] results in the composite tensile strength up to

119 MPa and modulus up to14 GPa. A flexural strength of 201 MPa and modules of 24 GPa were alsoachieved. MMSO offered additional methacrylate function groups, and hence the final compositesexhibited better mechanical properties with higher level of cross-linking. The extra addition of styrenein the bio-thermosets improved the mechanical (e.g., brittle) properties due to the low viscosity ofmolten styrene and resulted in a better fibre/matrix adhesion.

A study of the influences of weave architectures on the mechanical properties of flax fibre/bio-epoxycomposites was conducted by Adekunleet al. [75]. The woven fabrics were in the forms of plain, twilland dobby as shown in Figure 11. The tensile, flexural and impact properties were found to follow the

trend: Plain weave type < twill type < dobb type. The inherent thin weft yarns in plain weave fabricwere neglected, and hence the almost unidirectional properties along the warp yarn. TheT g offlax/bio-epoxy composites was around 85C from the tan plot, which was a little higher thanT g of70 C from the loss modulus data.

Figure 11. Weave architecture types: (A) plain; (B,C) two different twill; and (D) dobby [75].Adapted with permission from [17]. Copyright 2013 by SAGE.

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Figure 11. Cont.

3.5. Bio-Phenolic (Tannin) RESIN Based Flax Composites

Tannin resin, a natural phenolic resin, has been reinforced by flax fibres to offer environmental benefits and desirable characteristics aimed at reducing the environmental footprint of superlightelectric vehicle applications such as vehicle body panels, crash elements, side panels and bodytrims [92 94]. Tannin resins can be obtained from large varieties of plants (e.g., wattle, myrtle, pine etc. ) through the extraction with water and can be formed by crosslinking with formaldehyde andhexamine (Figure 12) [95]. The use of tannin as matrix has been paid more and more attention for thefollowing reasons: (i) the non-toxic nature of tannin and related hardeners; (ii) the wide availability oftannin and resulting cheaper cost; and (iii) the fast curing rate of tannin resins. Pizzi et al. [96] firstlymanufactured the mimosa tannin-based composites reinforced by a non-woven mat of flax fibres andalso studied their mechanical properties. There were two natural matrices used: (1) mimosa tannin with5% hexamine as hardener; (2) mixed tannin/ lignin resins in 50/50 solid content. The low density(8 mm thickness) and high density (1.2 mm thickness) composites were prepared, and followed bytensile tests, three point bending tests and Brinell hardness tests. The tensile strength was largelydependent on the variations of density. The low density composites had a 50% increase of tensilestrength from 536 kg/m3 to 727 kg/m3. By placing several layers together to obtain the requiredthickness, weak interfacial planes were unavoidable causing delamination rather easily.

Zhu et al. [94,97] studied flax/tannin composites with four fibre configurations of nonwoven matsand woven fabric lay-up angles ([0]8, [0, 90]2 and [0, +45, 90,45]2). A compression moulding process was conducted to manufacture 50 wt % flax containing tannin-based composites. Thelongitudinal tensile strength and Youngs modulus went up to 140 MPa and 9.6 GPa, respectively, atthe fibre form of [0]8. The relatively good fibre distribution in all directions of [0, +45, 90,45]2 composites gives rise to the best impact performance. The SEM image of the tensile fractured surfaceindicated poor flax/matrix adhesion in all woven flax composites, compared to nonwoven composites.The T g of around 60 C was observed for all the composite laminates. Thus there is a need to tailor the

fabric arrangement appropriately and to further enhance interfacial adhesion.

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Figure 12. The chemical reaction between tannin and hexamine: (a ) decomposition ofhexamine; and (b) polycondensation of condensed tannin [98].

3.6. Flax Composites Based on Other Matrices

In addition to PP, epoxy, PLA, bio-epoxy and tannin resin, a few other polymeric matrices for flaxcomposites are also available. Saiah and his co-workers [99] fabricated and characterisedflax-reinforced composites based on thermoplastics from wheat flour. The stress at failure increasedfrom 4.4 MPa at fibre content with 5 wt % to 8.9 MPa at 20 wt %, while there was an increment of270% in tensile modulus. They used X-ray diffraction to analyze the crystallinity of the flax fibre. The

increase in fibre content leads to an increase in intensity of peaks at 2

of 15.1, 16.8, 22.7 and 34.4,corresponding to the crystalline structure of flax fibres. In terms of thermal degradation, an observedmass loss peak in the 300 360 C range of the composites was found to increase with increasing fibre

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content. Andersons and Joffe [100] pointed out that the experimental tensile strength of a vinylesterresin/flax composite can reach the theoretically predicted values only at low fibre volume fraction (upto 20%) due to the somewhat misoreinted fibres caused by heterogeneous and short fibre length.

4. Nanotechnology Applied in Flax Composites

Nano-level technology has also been developing at a high speed in flax fibre polymer composites.The recent research activities and developments of nano-level cellulose fibre reinforced compositeswere reviewed by Eichhorn et al. [101]. With respect to the production of cellulose nanofibres andtheir composites, the review pointed out some issues that are worth noting: (1) structure damagemay occur during extraction of nanofibres; (2) lots of energy is required by mechanical ways toseparate fibres; and (3) the dispersion of nanofibres is crucial to control the composite quality and can be greatly enhanced by layer-by-layer deposition. Due to the high surface/volume ratio of cellulose

nanowhiskers, not only is a high efficiency of stress transfer reached but also chemical modificationlike grafting of DNA and coupling of chromophores can be applied for various purposes.

Flax bast fibres are one of the major sources to produce cellulose nanofibres, showing great potential for composite applications. Bhantnagar and Sain [102] investigated the flax fibre-derivedcellulose nanofibres with diameter range between 10 nm and 60 nm and prepared 10% nanofibrecontaining polyvinyl alcohol (PVA) composite films. The strong orientation of flax nanofibres givesrise to the high crystallinity of 59% obtained from the X-ray diffractograms. The tensile modulusincreased from 2.29 GPa of pure PVA to 6.1 GPa of PVA composites using 10% flax nanofibres. Theglass transition temperature of flax nanofibre/PVA composites was found to shift up to 58 C withincreasing fibre content from 5% to 40% [103]. Qua and his co-workers [104] found three degradationsteps of nanofibre/PVA composite films by tracking the weight change in TGA curves. The seconddegradation peak corresponding to the dehydration of PVA, was significantly increased by the additionof nanofibres due to the difficulty in breaking down the strong hydrogen bonding between PVA matrixand fibres.

Except for the nanofibre reinforcement, Huang and Netravali [19] added nano-clay particles to SPC(soy protein concentrated) composites reinforced by flax yarns and flax fabrics separately. The presence of nano-clay particles enhanced composite stiffness (e.g., tensile modulus and flexuralmodulus), but reduced the failure strain. The reduction of maximum strain may be due to the increasedrigidity of the polymer chain and defects caused by clay particles. The highest tensile strength andflexural strength of flax/SPC composites is 298 MPa and 117 MPa, respectively.

5. Conclusions

Polymer composites reinforced by flax fibres have attracted much attention from manufacturers andscientists due to the increased environmental awareness. Flax fibres with good mechanical properties(e.g., high tensile strength up to 1000 MPa) and physical properties have been reported asreinforcement for composites used for non-structural and structural applications. It is inevitable thatthe inherent detriments of flax, including moisture adsorption and incompatibility with some polymericsystems due to high hydrophilicity, present many challenges with respect to composite design and

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applications. Various chemical treatments, such as mercerization, silane treatment and benzoylation etc. , of flax fibres can improve the interface between fibres and matrix.

Flax fibre composites have a wide range of properties, depending on the matrix type, such asthermoplastic, thermosets and biomaterials. Flax/PP composite is the most commonly studiedcomposite and has been commercially used in automotive applications (e.g., vehicle panels).Anhydride treatment is a very efficient way to improve the flax/PP adhesion and hence the mechanical properties. PLA reinforced with flax gives fully biodegradable composites, ideal for replacement offlax/PP. The humidity sensitivity is still a problem as it decreases the long-term material properties.The properties of flax/epoxy composites are strongly influenced by the processing methods and fibreconfigurations. Resin transfer moulding and compression moulding are preferred for high performanceflax/epoxy composites. The bio-epoxy investigated in flax composites is mainly from soybean oil andthe final composites have similar independent factors to flax/epoxy composites. Mechanical performanceunder similar conditions could follow the fibre form trend: plain weave type < twill type < dobb type.Flax/tannin composites have been studied only in the recent 2 3 years, mostly for automotiveapplications. The ease of processing, good mechanical properties and low overall cost have spurred onfurther development of flax/tannin composites. Also, the use of nanotechnology (flax nanofibres andthe addition of nanoclays in flax composites) highly improves the mechanical performances.

Acknowledgments

The authors are thankful to European Commission Framework 7for the financial support throughProject title Development of new light high-performance environmentally benign composites made of bio-materials and bio-resins for electric car (ECOSHELL), EC FP7 Project No. 265838.

Conflicts of Interest

The authors declare no conflict of interest.

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